A study of the hypothesis that Attention Deficit Hyperactivity Disorder (ADHD) is a genetically influenced brain disorder will be undertaken through a genome wide search for loci linked to ADHD. To this end, 700 affected sibling pairs and 100 densely affected multi-generation Hispanic families will be recruited. Participating individuals will undergo a battery of psychological tests and will have blood drawn for the linkage analysis and positional cloning studies that will be used to search for genes associated with ADHD.